Nitriding of steels by Plasma Immersion Ion Implantation (PI3) allows access to a large process parameter space. Although the parameters associated with the high-energy ion bombardment (implantation energy, high-voltage pulse length and frequency, ion current density and time-averaged dose rate) are important, the treatment temperature and plasma parameters such as ion density, excited neutral density and plasma potential also play a vital role. Previous investigations have been hampered by the use of the high-energy ion bombardment to heat the workpiece. In this paper, we present the results of a study in which the treatment temperature and the ion bombardment were decoupled by radiatively heating. The effect of varying high-voltage pulse length, repetition rate, total implanted dose, plasma density and potential on the nitrogen uptake during PI3 treatment depends strongly on whether nitrides are formed in the surface (e.g. Ck45 mild steel) or nitrogen is incorporated in solid solution (e.g. X6CrNiTi 1810 austenitic stainless steel). In the first case, nitride formation can be suppressed by increasing the high-voltage pulse frequency or can be enhanced by treating at a high pressure or plasma potential. In the second case, the thickness of the modified layer can be increased by increasing the ion current density or time-averaged dose rate. In both cases, nitrogen uptake by direct thermochemical absorption from the plasma is significant.